In the United States and many other countries, heart disease is the leading cause of death and disability. One particular kind of heart disease is atherosclerosis, which involves the degeneration of the walls and lumen of the artery walls throughout the body. Scientific studies have demonstrated the thickening of the arterial wall and eventual encroachment, of the tissue into the lumen as fatty material is built up. This material is known as "plaque." As the plaque builds up and the lumen narrows, blood flow is restricted. If the artery narrows too much, or if a blood clot forms at an injured plaque site (lesion), flow is severely reduced, or cut off and consequently the muscle that it supports may be injured or die due to a lack of oxygen. Atherosclerosis can occur throughout the human body, but it is most life threatening when it involves the coronary arteries which supply oxygen to the heart. If blood flow to heart muscle is significantly reduced or cut off, a myocarcial infarction or "heart attack" often occurs. If not treated immediately a heart attack frequently leads to death.
The medical profession relies upon a wide variety of tools to treat coronary disease, ranging from drugs to open heart "bypass" surgery. Often, a 9 lesion can be diagnosed and treated with minimal intervention using catheter-based tools threaded into the coronary arteries via the femoral artery in the groin. For example, one treatment for lesions is a procedure known as percutaneous transluminal coronary angioplasty (PTCA) whereby a catheter with an expandable balloon at its tip is threaded into the lesion and inflated. The underlying lesion is re-shaped, and hopefully, the lumen diameter is increased to restore blood flow.
The practiced method for guiding a catheter during procedures such as PTCA is real time X-ray images. With this method, a radiopaque dye is injected into the coronary tree to provide a map of blood flow. This technique helps a physician to identify sites where blood flow is restricted. After identifying these sites, therapeutic devices are positioned using a live X-ray image. However, the X-ray image does not give information about the morphology, i.e., form and structure, of the artery.
In the last 10 years, cardiologists have adopted a new technique to obtain information about the coronary vessel and to help view the effects of therapy on the form and structure of the vessel and not just the blood flow. This technique, known as Intracoronary or Intravascular Ultrasound (ICUS/IVUS) employs miniaturized transducers on the tip of the catheter which provide electronic signals to an external imaging system in order to produce a two or three-dimensional image of the lumen, the arterial tissue, and tissue surrounding the artery. These images are generated in substantially real time and have a high degree of resolution. As an improvement over X-ray imaging, the transducers facilitate the construction of images of the exact site where the transducers are placed. within the vessel.
Several ICUS/IVUS devices are now commercially available for sale in the United States and. other countries. These devices include a transducer probe assembly having either a solid state transducer array or a rotating crystal. The physician is most interested in identifying the size and shape of the lumen, and any flaps or tears in the plaque. Commercially available systems produce detailed images of these relatively static features due to the relatively high frequency of ultrasound they employ. Image signals are typically transmitted at frequencies between 10 and 40 MHz.
As previously explained in O'Donnell et al. U.S. Pat. No. 5,453,575, there is a common problem associated with these devices operating at such high frequencies. As the frequency of the ultrasound is raised, the backscatter from blood increases as the fourth power of the frequency. At frequencies of around 30 MHz, the amplitude of the backscatter from blood approaches the amplitude of the backscatter and reflections from arterial tissue. Because of this phenomenon, the image of the lumen is filled with blood echoes, and it is often difficult to delineate blood from surrounding tissue. Therefore, the physician has trouble defining the lumen.
The problem of blood echoes has been addressed in a number of different manners imaging dynamic regions in a field of view. An example of such a system and method is provided in O'Donnell et al. U.S. Pat. No. 5,453,575 wherein a "dynamic" image is generated and thereafter superimposed upon a second image representing relatively static features of a field of view in a vasculature.
While the known imaging systems and methods helped distinguish dynamic and static features in a field of view during intravascular imaging, certain shortcomings were encountered. First, the known intravascular blood flow imaging systems and methods tend to present slow moving tissue as a dynamic region which cannot easily be distinguished from regions of moving blood. Further, present ultrasound vascular imaging systems tend to present an unstable image wherein dynamic portions of the image change drastically from displayed frame-to-frame thereby creating distracting "flashing" displays. In other words, the color bits for a large percentage of pixels on the screen toggle between on and off states when a display is refreshed with new image data. The color assigned to particular pixels exhibits similar instabilities.